The present invention relates to a technique for manufacturing a semiconductor integrated circuit device, and more particularly to a technique which is effective in a method of forming an element isolation groove in a semiconductor substrate.
A technique of LOCOS (Local Oxidation of Silicon) is generally known as an element isolation technique, and developments of new element isolation techniques substitutable for the technique of the LOCOS have been promoted in accordance with downsizing of a semiconductor element.
The following excellent advantages can be obtained by an element isolation groove formed by embedding an insulating film such as a silicon oxide film or the like in a groove formed in a semiconductor substrate.
(a) The element isolation interval between the element isolation grooves can be reduced.
(b) Control of the film thickness of an element isolation film can be facilitated and setting of field inversion voltages can be easily carried out.
(c) An anti-inversion layer can be separated from a diffusion layer and a channel region for an element by separately injecting impurities to the side wall and the bottom of the groove, so that subthreshold characteristics can be maintained and connection leakage and back-gate effects can be advantageously reduced.
To form an element isolation groove in a semiconductor substrate, a semiconductor substrate is subjected to a heat treatment to form a thin silicon oxide film (or pad oxide film) The pad oxide film is formed for the purpose of relaxing a stress onto the substrate when sintering a silicon oxide film embedded in the groove in a later step, and for the purpose of protecting an active region when removing a silicon nitride film used for a mask for oxidation.
Next, a silicon nitride film is deposited on the pad oxide film by a CVD (Chemical Vapor Deposition) method, to remove a silicon nitride film from an element isolation region by etching with a mask of a photoresist. Since it is characteristic of a silicon nitride film to be hardly oxidized, a silicon nitride film is used as a mask for preventing oxidation of the surface of a substrate below the film. In addition, a silicon nitride film is used as a mask when etching a substrate to form a groove.
Next, a groove is formed in a semiconductor substrate by etching with a silicon nitride film as a mask, and thereafter the substrate is oxidized in a vapor atmosphere at 1000.degree. C. or more, to form a thin silicon oxide film on the inner wall of the groove. The silicon oxide film is formed for the purpose of eliminating etching damages caused on the inner walls of the groove, and for the purpose of relaxing a stress onto a silicon oxide film which is embedded in the groove in a later step.
Subsequently, a silicon oxide film is embedded in the groove by depositing a silicon oxide film on the semiconductor substrate by a CVD method, and thereafter the silicon oxide film embedded in the groove is subjected to sintering. The sintering is a step for improving the film quality of the silicon oxide film embedded in the groove.
Next, a chemical mechanical polishing method or the like is used to remove the portion of the silicon oxide film above the silicon nitride film, so that the silicon oxide film remains only in the groove. Thus, an element isolation groove filled with a silicon oxide film is formed. Thereafter, the silicon nitride film used as a mask for oxidation is removed by etching, and further, a semiconductor element is formed in an active region.
If a shoulder portion of the element isolation groove formed by the method described above is angular, the gate oxide film which is formed above the element isolation groove in a later step is locally thinned at the shoulder portion, resulting in a problem (or hump characteristic) that a drain current flows even with a low voltage. Therefore, a proposal has been made as for a technique for rounding the shoulder portion of the element isolation groove.
Although wet or steam oxidation is said to be effective to sinter the silicon oxide film embedded in the groove, the inside (or the side wall in particular) of the groove is easily oxidized if the wet oxidation is carried out (i.e., the bottom of the groove is relatively hard to be oxidized because oxidation starts from the surface of the groove). If the side wall is thus oxidized, there appears a problem that an active region is narrowed. In addition, if the oxide film is thick, a large stress is generated at the interface with the substrate and causes a problem that the shoulder portion which is once rounded becomes angular again. Therefore, a proposal has been made as for a technique of covering the inner wall of the groove with a silicon nitride film to prevent the side wall of the groove from being oxidized.
Japanese Patent Laid-Open No. 2-260660 discloses a technique in which a groove is formed at an element isolation region in a semiconductor substrate by etching with use of masks of a thermal oxide film and a silicon nitride film which are formed on the surface of an active region of the semiconductor substrate, and thereafter a pad oxide film exposed from the end surface of a groove is wet-etched to be recessed backward by 0.1 .mu.m so that a birds beak more easily enters during oxidation. Thereafter, a thermal oxide film is formed on the inner wall of the groove to round the shoulder portion of the groove. According to this method, a parasitic channel effect can be restricted by rounding the shoulder portion of the groove, so that a MOS integrated circuit having an excellent cut-off characteristic can be obtained.
In a groove forming method described in Japanese Patent Laid-Open No. 4-303942, a groove is formed at an element isolation region of a semiconductor substrate by etching with use of masks of a thermal oxide film, a silicon nitride film, and a PSG film formed on the surface of an active region of a semiconductor substrate, and thereafter the PSG film is removed by wet-etching (in which the thermal oxide film below the silicon nitride film is undercut by about 500 to 1000 angstrom). Next, the semiconductor substrate is oxidized to form a thermal oxide film on bottom, side wall, and undercut portions, thereby rounding the shoulder portion. Thereafter, an insulating film is embedded in the groove. According to this method, the undercut portion is filled with a thermal oxide film, thereby eliminating concave portions. Thus, an insulating film is completely embedded in the groove and formation of voids can be prevented.
A groove forming method described in Japanese Patent Laid-Open No. 8-97277 is as follows. Firstly, a groove is formed in a substrate, and a thermal oxide film is formed on the inner surface (including surfaces of side walls and bottom) of the groove. A silicon nitride film is formed on the thermal oxide film, and a silicon film (which is of amorphous, or polycrystal silicon, or monocrystal silicon) is further formed on the silicon nitride film. Thereafter, the inside of the groove is filled with a silicon oxide film and the surface thereof is flattened. A silicon oxide film is deposited on the entire surface of the substrate, and thereafter the silicon film is oxidized in an oxidation atmosphere containing vapor at about 950.degree. C. and changed into a silicon oxide film, before carrying out flattening. In this state, the silicon substrate is protected by the silicon nitride film and is therefore not oxidized. According to this method, a silicon oxide film can be embedded without causing voids by forming a thin film (e.g., a silicon film) on the inner surface of the groove which has good compatibility with a silicon oxide film. Although the silicon film in the groove must be oxidized and changed into a silicon oxide film later, the silicon substrate is not oxidized when oxidizing the silicon film since a silicon nitride film is provided between the silicon film and the substrate. Therefore, the element characteristic is not deteriorated.